In the construction of electrochemical fuel cells it is desirable to use low cost construction components, whenever possible. This is not always a simple thing to achieve because in many instances, especially in acid-electrolyte fuel cells, the electrolyte is extremely corrosive and thereby attacks many common construction materials.
An electrochemical fuel cell is a device for converting chemical energy of a fuel directly into electrical energy by means of a galvanic process. The components of a fuel cell consist of an electrolyte which conducts ions between two electrodes, a positive or reducing electrode, and a negative or oxidizing electrode.
The negative electrode is sometimes referred to as the fuel electrode and the positive electrode is most commonly an oxygen or air electrode. The electrodes are brought in intimate contact with the electrolyte where galvanic reactions take place when the external circuit is closed. It is next necessary to be able to remove and transfer electrons to and from the electrodes by means of appropriate conductors. Simultaneously it is necessary to be able to supply fuel and oxygen to the electrode to provide a continuous source of the active material. Therefore, in addition to the electrodes themselves, there must be provided access to the active site via the fuel and oxidizing gas compartments in the cell.
In many types of fuel cells, the electrodes are supported by metallic grids. The metallic grids, in addition to being mechanical supports, act as electronic current conductors as well as providing porous structures through which gas can readily pass. In the case of acid fuel cells, which are an important embodiment of the present invention, the choice of electronic conductors to be used as metallic grids in the electrode assemblies is severely limited by the corrosive nature of the acids. Only a very few metallic materials are capable of withstanding the corrosive action of concentrated sulfuric and phosphoric acids commonly used in fuel cells. Typical metals which can provide the above function are platinum, tantalum, and sometimes niobium if the temperatures are not too high.
Metal alloys which might be compatible with the acid are generally poor materials for use in fuel cells because of severe oxidizing and reducing conditions that exist at the electrode during the fuel cell operation.
We have discovered that graphitic materials are ideally suited to withstand the corrosive nature of phosphoric and sulfuric acids over the entire range of acid concentration and operating temperatures commonly used in fuel cells. This application is directed to solving the problem of how to produce the graphitic materials in an appropriate form to provide the additional functions of gas accessibility and current collection as well as mechanical electrode support.
The four main requirements for an electrode substrate in the fuel cell are good electrical conductivity, good gas diffusion characteristics, resistance to the electrolyte from a corrosion viewpoint, and good thermal conductivity. The latter characteristic is almost invariably met when the higher priority electrical conductivity criterion is met.